This application is a 371 of PCT/JP01/05782 filed Jul. 4, 2001.
The present invention relates to a debindering method used for removing a binder from a powder molded body containing a binder consisting of at least two kinds of binder components.
When a ceramic powder or metal powder molded body is produced by extrusion, injection or the like, a binder is added into a ceramic powder or a metal powder in order to obtain higher moldability. The binder component used is determined in view of the shape, raw material, molding method, etc. of the molded body to be produced, and it is not rare to use a binder consisting of at least two components.
In producing, for example, a molded body of honeycomb form used in production of, for example, a carrier for exhaust gas purification catalyst or a filter for diesel particulate, there has come to be used a thermoplastic binder which is a mixture of a water-insoluble wax and a thermoplastic resin, in place of a conventional binder which is a water-soluble thermosetting methyl cellulose, because the partition walls of honeycomb structure have become thinner (25 to 100 xcexcm) and consequently higher fluidity during extrusion and higher shape retainability after extrusion have become necessary.
Such a thermoplastic binder contains no water therein and therefore requires no drying step unlike the case using a water-soluble binder; however, the weight ratio of binder component in molded body is inevitably large. Therefore, a large amount of a binder must be removed from the molded body during the debindering step, and it is an important task to conduct a debindering treatment rapidly without generating any defect in the molded body.
For debindering of molded body, it is generally conducted to heat a molded body to allow the binder in the molded body to vaporize and decompose thermally. This debindering method has been used also in debindering of the above-mentioned molded body of honeycomb form. Meanwhile, a thermoplastic binder has been used in producing a molded body by injection and, for debindering of the produced molded body, a method is known in which part of the binder used is removed by dissolution (extraction) using an organic solvent and then the remaining binder is removed by heating (see JP-B-59-27743).
In the method for debindering of molded body by heating a molded body to allow the binder contained therein, to vaporize and decompose thermally, however, the thermal decomposition of organic binder generates a large amount of a decomposition gas (e.g. CO2) and accordingly gives a large burden to the environment. Further, the abrupt heat generation during the thermal decomposition of binder gives rise to a temperature difference inside the molded body; and the thermal stress caused by this temperature difference, plus the pressure of decomposition gas or the dimensional change caused by decomposition tend to generate defects such as cracks and the like particularly in a molded body of relatively low strength, such as honeycomb molded body of thin partition walls. In order to prevent such defects, it is necessary to slow the temperature elevation during the heating of molded body and make small the heat generation inside the molded body, which makes long the debindering time.
In the method for debindering of molded body by extracting the binder contained in a molded body, with an organic solvent, there is used an organic solvent which gives adverse effects on human health and the environment and which may cause fire; this invites the deterioration of working environmental, reduced safety and an increase in the incidental facility required for emission gas treatment, etc.
The present invention has been made in view of the above-mentioned situation and aims at providing a method for debindering of powder molded body, which enables a short time and rapid debindering treatment while preventing the generation of defects such as cracks and the like, which has high safety to human health and environment, and which requires a low facility cost.
According to the present invention, there is provided a method for debindering of powder molded body, characterized by dipping, in an extracting solution composed of an aqueous surfactant solution, a ceramic powder or metal powder molded body containing a binder comprising at least two kinds of binder components, to selectively extract and remove at least one kind of binder component from the molded body, and then removing the binder components remaining in the molded body after extraction.
In the present invention, it is desirable from the standpoint of preventing the cracks generating during the extraction from the molded body that at least 40% by volume of the binder component to be selectively extracted is extracted and removed before the linear expansion of the molded body during the extraction step reaches 50% relative to the linear expansion of the molded body when no extraction and removal is made. Even if the linear expansion of the molded body during the extraction step is 50% or more relative to the linear expansion of the molded body when no extraction and removal is made, it is desirable to allow the molded body to make uniform expansion from the standpoint of preventing the cracks generating during the extraction from the molded body. It is preferred to selectively extract and remove at least one kind of binder component from the molded body, wash the resulting molded body with water, remove the water remaining in or adhering to the molded body, and then heat the molded body to remove the binder components remaining in the molded body without being extracted by the extracting solution, because this operation makes easy the removal of water from the molded body and prevents the generation of cracks in the molded body during its drying.